1. Field of the Invention
The invention relates to a sludge treatment plant, more particularly, for the hygienization thereof.
2. Description of the Related Art
Many industrial or urban operations generate waste that have to undergo a variety of treatments for various purposes. Wastes from industrial or urban effluent treatment plants or water treatment plants can be cited as examples. These wastes comprise masses composed of biological and physico-chemical sludge.
This type of wastes has to undergo more or less complex treatments, for economical and/or ecological purposes. In addition, some laws set clear and obligatory standards for the treatment of such wastes.
This results in waste treatment being now both widespread and diversified.
The “sludge” part of wastes is a sensitive sector in the field of wastes treatment. Thus, the treatment of sludge for the reuse thereof conventionally comprises thickening, dewatering and drying if necessary, in some installations.
Sludge drying appears as an important step since it more particularly makes it possible to significantly reduce the volume of sludge. A small volume is important as regards storage, transportation and/or disposal.
It should be understood that “dried sludge” means sludge the moisture content of which does not exceed about 30%. As a matter of fact, it is important to obtain a compact sludge product the integral structure of which is not damaged and keeps a granular appearance. Some left moisture is appropriate. In fact, too dry a sludge product may contain a large amount of dusty materials. Any form of dust is generally unsuitable for transport, storage or reclaiming.
So-called thermal drying facilities which implement air drying in a closed circuit comprising a moisture exhaust are more particularly known. Solar drying facilities are also known which have an advantage in terms of investment and operation costs that are much more attractive than those of thermal drying.
To optimize the implementation or to save energy, the state of the art provides for various drying facilities and/or processes.
Document WO 2004/020922 A1 discloses a device for drying and turning sludge. To avoid forced ventilation positioned on the greenhouse roof, the device comprises frame-mounted driven fans that are directly positioned on means ensuring the turning of sludge. The fans ensure the renewal of air in the greenhouse.
Document EP 0413940 A discloses an installation comprising a pressing device for dewatering sludge and thus improving the drying.
But the sludge treated by the facilities of the prior art is not directly compatible with a use in critical technical fields as regards health.
Other fields, such as agriculture, require a strict spreading plan to be able to reclaim the sludge generated by drying facilities. As a matter of fact, the sludge generated by drying facilities and used in agriculture is treated as “waste”. A risk management procedure aiming at the capacity of a system to self-purify not being exceeded should therefore be provided for. The implementation of this management procedure is sometimes complex.
The amount of uni- or multicellular organisms present in the sludge must be checked ahead to avoid spreading plans. This includes checking the amounts of pathogenic agents, bacteria, yeasts, fungi, including molds, multicellular organisms and eggs thereof.
This is the reason why some sludge undergoes a so-called hygienization treatment to extend their scope of application after a simple drying. This hygienization takes place after drying in separate facilities. The aim is to provide so-called approved sludge as a fertilizer material in the end. Such approved sludge is ready for use in sensitive technical fields or in agriculture, without strict spreading plans having to be provided for.
The agricultural sector is experiencing the problem of pathogenic agents. The most common and difficult to eliminate agents are:                tThe bacteria of the Enterobacteriaceae family such as Escherichia coli or of the Salmonella type;        the bacteria of the Clostridiaceae family such as Clostridium perfringens;         the bacteria of the Enterococcaceae family such as Enterococcus faecalis;         the bacteria of the Staphylococcaceae family such as Staphylococcus aureus;         The bacteria of the Listeriaceae family such as Listeria monocytogenes;         the eggs of Nematoida (or Nematoda); or        the virus of the Enterovirus type.        
To eliminate these pathogenic agents from dried sludge, the state of the art provides methods which are generally difficult to implement and/or involve high investment and processing costs.
Document EP 1 621 523 discloses a method using high temperature static and thermal heating. The document also mentions hygienization of sludge at 50° C. for 1 month.
Document FR 2 904 824 describes a microwave hygienization treatment.
It should be noted that the energy balance increases with the complexity of the process implemented. It is therefore important to find a balance between as simple as possible an implementation and a satisfactory energy balance, while taking into account the imposed health regulations.
In addition, the techniques of the prior art do not enable a satisfactory elimination of the bacteria of the Clostridiaceae family.